1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method of removing a photoresist layer.
2. Brief Description of the Related Art
A typical ion implantation process utilizes a photoresist layer as an ion implantation mask when fabricating a semiconductor device. For instance, a dual poly gate process uses a photoresist layer as an ion implantation mask.
FIGS. 1A to 1C are cross-sectional views showing a typical dual poly gate process.
Referring to FIG. 1A, a substrate 11 is defined into an N-channel metal-oxide semiconductor (NMOS) region and a P-channel metal-oxide semiconductor (PMOS) region. A device isolation structure 12 is formed in the substrate 11. A gate oxide layer 13 is formed over the substrate 11 and the device isolation structure 12. A gate polysilicon layer 14A and an N-type doped polysilicon layer 14 are formed over the gate oxide layer 13. In more detail, a gate polysilicon material layer is formed over the gate oxide layer 13. N-type impurities are implanted into a portion of the gate polysilicon material layer in the NMOS region by an ion implantation process N+ IMP using a first photoresist pattern 15 to form the N-type doped polysilicon layer 14A. The first photoresist pattern 15 exposes the NMOS region and covers the PMOS region. A remaining portion of the gate polysilicon material layer in the PMOS region is referred to as the gate polysilicon layer 14.
Referring to FIG. 1B, the first photoresist pattern 15 is removed. A photoresist layer is formed over the resultant substrate structure. The photoresist layer is patterned by performing photo-exposure and developing processes to form a second photoresist pattern 16. The second photoresist pattern 16 exposes the PMOS region and covers the NMOS region. P-type impurities are implanted into the gate polysilicon layer 14 in the PMOS region by an ion implantation process P+ IMP using the second photoresist pattern 16 to form P-type doped polysilicon layer 14B.
Referring to FIG. 1C, the second photoresist pattern 16 is removed. Tungsten silicide layers 17 are formed over the resultant substrate structure. A gate patterning process is performed onto the substrate structure to form an N+poly gate 14C in the NMOS region and a P+ poly gate 14D in the PMOS region. The N+ poly gate 14C includes N-type doped polysilicon and the P+ poly gate 14D includes P-type doped polysilicon.
In the aforementioned typical method, different impurities, i.e., phosphorus (P) and boron (B), are implanted into the gate polysilicon material layer to embody a dual poly gate configured with the N+ poly gate 14C and the P+ poly gate 14D. The impurities are implanted using a high density ion implantation process at high dose ranging from approximately 1×1015 cm−2 to approximately 1×1016 cm−2. In the typical method, a gas including oxygen and nitrogen (O2/N2 chemistry) is used to remove the first and the second photoresist patterns 15 and 16 after the ion implantation process is performed.
However, the high density ion implantation process at high dose causes substantial hardening of the first and the second photoresist patterns 15 and 16. Thus, the first and the second photoresist patterns 15 and 16 may not be removed easily. Photoresist residues may remain after the removal of the first and the second photoresist patterns 15 and 16. The oxygen (O2) used during the removal of the first and the second photoresist patterns 15 and 16 reacts with impurities existing in the first and the second photoresist patterns 15 and 16, i.e., arsenic (As), phosphorus (P), and boron (B), to form an impurity oxide layer, e.g., As2O3, P4O6, and B2O3, covering surfaces of the first and the second photoresist patterns 15 and 16.
Thus, abnormal oxidation may occur in the tungsten silicide layers 17 during a subsequent process due to the remaining photoresist residues. Also, an interface defect may occur between the poly gates and the tungsten silicide layers may generate a source of lifting or particle after the gate patterning is performed. Photoresist may not be completely removed when a doping level of an ion implantation process is high. The process time lengthens in order to remove the remaining photoresist, and thus, mass-producibility decreases.
FIG. 1D is a micrographic view of photoresist residues generated by the typical method.